This invention relates to an automatic focusing apparatus for use in a video camera, electronic still camera and so on, and more particularly to an automatic focusing apparatus suitable for use in a TTL (Through The Lens) imaging system in which a high frequency band component (focal voltage) is derived from an image signal and the lens position is controlled such that the maximum of the focal voltage can be obtained.
As disclosed in, for example, JP-A-60-42723 or National Technical Report, Vol 31, No. 6, Dec., 1985, pp 65-67, a TTL imaging type automatic focusing apparatus has been proposed which has a feedback circuit wherein a high frequency band component signal is extracted from an image signal produced from an image sensing device while the optical path length between an object to be imaged and the light receiving surface of the image sensing device is being varied slightly by using a signal of a predetermined reference frequency, a frequency component signal representative of a slight variation is derived from the high frequency band component signal, the frequency component signal is compared with the reference frequency signal for slight variation of the optical path length, an in-focus direction is determined on the basis of a signal obtained from the comparison, and a focusing lens is moved in the in-focus direction to obtain the maximum value of the high frequency band component signal. According to the aforementioned JP-A-60-42723, the optical path length can be varied to a small extent in a way in which a vibratory prism disposed in front of the image sensing device and mounted with a piezoelectric element is vibrated under the application of an electric signal of a predetermined reference frequency to the piezoelectric element to slightly change the optical path length in the optical axis direction of the lens system or in another way in which a lens other than a focusing lens is vibrated by means of a piezoelectric element to slightly change the optical path length. An example specifically practiced in the latter way is described in the literature.
In general, a zoom lens system 1 in a video camera and the like basically comprises, as shown in FIG. 3, a focusing lens (front or objective lens) means 2, a variator lens means 3, a compensator lens means 4, an aperture unit 5 and an image forming lens (master lens) means 6. As well known in the art, of the basic construction, the focusing lens means 2 has the function of focusing a desired object which lies at an arbitrary distance, the variator lens means 3 has the function of changing magnification for zooming, the compensator lens means 4 is movable for zooming and has the function of correcting an error in focusing the desired object during zooming, and the image forming lens group 6 has the function of forming an optical image on an image sensing device 7.
Thus, according to the example in the aforementioned literature, either the prism provided separately from the lens system 1 of the above basic construction and disposed in front of the image sensing device 7 or the image forming lens 6 is vibrated by means of the piezoelectric element to slightly vary the optical path length.
As is clear from the above, the prior art employs a lens focus matching unit inserted in the feedback loop and therefore an automatic focusing apparatus of excellent in-focus accuracy can advantageously be realized with the lens focus matching unit even when the lens focus matching unit is assembled with less accuracy and is of rough mechanical accuracy. However, the prior art particularly requires the piezoelectric element for a mechanism adapted to slightly vibrate the optical path length, raising a problem that the mechanism must be contrived structurally in point of, for example, mounting and supporting in order to obtain long-term stability. Further, a drive voltage of several of tens of volts or more is required for driving the piezo-electric element, raising another problem that an instrument such as a video camera designed to operate at a low voltage of battery must include an additional means for generating such a relatively high voltage.
JP-A-55-76309 discloses another automatic focusing apparatus which can dispense with the piezoelectric element having the above disadvantages to move a focusing or objective lens in the in-focus direction while keeping the focusing or objective lens vibrated slightly.
In particular, according to this prior art, a focusing or objective lens means is moved by means of a motor while being vibrated slightly and the separate provision of the mechanism for slight vibration of the optical path length as described previously is unneeded, leading to presumed simplification of construction. However, a DC motor is generally used for focus driving and it is very difficult for this type of motor to slightly vibrate the relatively weighty focusing or objective lens means to a visually undetected degree under the influence of large inertia of the focusing or objective lens means and in addition, it is also very difficult from the standpoint of motor life to practically cause the motor to unceasingly vibrate or rotate alternately clockwise and counterclockwise.